1. Field of the Invention
The present invention relates to a polarization-mode changeable or selective semiconductor laser whose polarization mode of output light can be changed between different polarization modes (typically, a transverse electric (TE) mode and a transverse magnetic (TM) mode) by an external control, an apparatus including the semiconductor laser, and so forth.
2. Related Background Art
Conventionally, Japanese Patent Laid-Open Application No. 2-159781 discloses an example of a distributed feedback (DFB) laser whose polarization mode of output light can be varied by an external control. In the DFB semiconductor laser, carriers are injected to perform phase adjustment and effect population inversion, and by this means the phase of internal propagating light is varied. Thus, light oscillation occurs in either the TE mode or the TM mode, whichever has a lower threshold gain in accordance with the phase condition.
In the structure of that DFB semiconductor laser, an active layer is formed of a bulk material such that the difference in gain between those different polarization modes is reduced, and the pitch of its diffraction grating is precisely set such that its Bragg wavelength is coincident with the gain peak wavelength. As a result, oscillation competition between those polarization modes can be achieved in the semiconductor laser by controlling amounts of carriers injected into its front and rear regions. In this structure, however, a desired competitive condition between those polarization modes can not be attained unless the pitch of the diffraction grating is accurately and precisely set, and hence yield of the polarization-mode selective device is likely to be undesirably low.
Further, Japanese Patent Laid-Open Application No. 7-235718 discloses another conventional device in which there are serially arranged a plurality of DFB regions whose dominant polarization modes are respectively the TE mode and the TM mode. In this structure, there is arranged an active layer (typically, a quantum well structure) which exhibits different gain spectra for the two different polarization modes, and pitches of the diffraction gratings are independently set in those TE and TM regions such that the Bragg wavelengths for those two polarization modes are respectively accorded to gain peak wavelengths for those two polarization modes in those TE and TM regions. However, its fabrication process is complicated since the different pitches are separately set in those two regions, and accordingly its yield is likely to be too low as well.
Furthermore, although the waveguide structure and the active layer may be formed differently as between the two regions to form the above-discussed TE and TM regions with the pitches of their diffraction gratings being the same, this requires more complicated epitaxial growth and processing than the above process of forming the different pitches. Thus, it is hard to obtain favorable polarization-mode switching characteristics with good reproducibility.